Exercise:
Derive a predictive formula of chest circumference from body height and body weight according to the data (chest circumference, body height, and body weight of military academy students) that Healy used to explain multiple linear regression analysis (Y=a+b1X1+b2X2+b3X3+b4X4…….).
（MJR Healy, Arch Dis Child 1995; 73: 177）
When age (dummy number) is entered, the t tests will show that age coefficient is not statistically significant. Predict mean population value of Y at X1=175 m and X2=75 kg.
